Thiswemustlikewiseassumetohappeninthecaseofqualitativechange;forthatpartwhich[forexample]hasbeenheatedbysomethinghot，heats[inturn]thepartnexttoit，andthispropagatestheaffectioncontinuouslyonwardsuntiltheprocesshascomeroundtoitsointoforigination.Thismustalsohappenintheorganwhereintheexerciseofsense—perceptiontakesplace，sincesense—perception，asrealizedinactualperceiving，isamodeofqualitativechange.Thisexplainswhytheaffectioncontinuesinthesensoryorgans，bothintheirdeeperandintheirmoresuperficialparts，notmerelywhiletheyareactuallyengagedinperceiving，butevenaftertheyhaveceasedtodoso.Thattheydothis，indeed，isobviousincaseswherewecontinueforsometimeengagedinaparticularformofperception，forthen，whenweshiftthesceneofourperceptiveactivity，thepreviousaffectionremains;forinstance，whenwehaveturnedourgazefromsunlightintodarkness.Fortheresultofthisisthatoneseesnothing，owingtotheexcitedbythelightstillsubsistinginoureyes.Also，whenwehavelookedsteadilyforalongwhileatonecolour，e.g.atwhiteorgreen，thattowhichwenexttransferourgazeappearstobeofthesamecolour.Againif，afterhavinglookedatthesunorsomeotherbrilliantobject，weclosetheeyes，then，ifwewatchcarefully，itappearsinarightlinewiththedirectionofvision（whateverthismaybe），atfirstinitsowncolour;thenitchangestocrimson，nexttopurple，untilitbecomesblackanddisappears.Andalsowhenpersonsturnawayfromlookingatobjectsinmotion，e.g.rivers，andespeciallythosewhichflowveryrapidly，theyfindthatthevisualstimulationsstillpresentthemselves，forthethingsreallyatrestarethenseenmoving：personsbecomeverydeafafterhearingloudnoises，andaftersmellingverystrongodourstheirpowerofsmellingisimpaired;
andsimilarlyinothercases.Thesephenomenamanifestlytakeplaceinthewayabovedescribed.
Thatthesensoryorgansareacutelysensitivetoevenaslightqualitativedifference[intheirobjects]isshownbywhathappensinthecaseofmirrors;asubjecttowhich，eventakingitindependently，onemightdevotecloseconsiderationandinquiry.Atthesametimeitbecomesplainfromthemthatastheeye[inseeing]
isaffected[bytheobjectseen]，soalsoitproducesacertaineffectuponit.Ifawomanchancesduringhermenstrualperiodtolookintoahighlypolishedmirror，thesurfaceofitwillgrowcloudywithablood—colouredhaze.Itisveryhardtoremovethisstainfromanewmirror，buteasiertoremovefromanoldermirror.Aswehavesaidbefore，thecauseofthisliesinthefactthatintheactofsightthereoccursnotonlyapassioninthesenseorganactedonbythepolishedsurface，buttheorgan，asanagent，alsoproducesanaction，asispropertoabrilliantobject.Forsightisthepropertyofanorganpossessingbrillianceandcolour.Theeyes，therefore，havetheirproperactionashaveotherpartsofthebody.Becauseitisnaturaltotheeyetobefilledwithblood—vessels，awoman\'seyes，duringtheperiodofmenstrualfluxandinflammation，willundergoachange，althoughherhusbandwillnotnotethissincehisseedisofthesamenatureasthatofhiswife.Thesurroundingatmosphere，throughwhichoperatestheactionofsight，andwhichsurroundsthemirroralso，willundergoachangeofthesamesortthatoccurredshortlybeforeinthewoman\'seyes，andhencethesurfaceofthemirrorislikewiseaffected.Andasinthecaseofagarment，thecleaneritisthemorequicklyitissoiled，sothesameholdstrueinthecaseofthemirror.Foranythingthatiscleanwillshowquiteclearlyastainthatitchancestoreceive，andthecleanestobjectshowsupeventheslighteststain.Abronzemirror，becauseofitsshininess，isespeciallysensitivetoanysortofcontact（themovementofthesurroundingairactsuponitlikearubbingorpressingorwiping）;onthataccount，therefore，whatiscleanwillshowupclearlytheslightesttouchonitssurface.Itishardtocleansesmudgesoffnewmirrorsbecausethestainpenetratesdeeplyandissuffusedtoallparts;itpenetratesdeeplybecausethemirrorisnotadensemedium，andissuffusedwidelybecauseofthesmoothnessoftheobject.Ontheotherhand，inthecaseofoldmirrors，stainsdonotremainbecausetheydonotpenetratedeeply，butonlysmudgethesurface.
Fromthisthereforeitisplainthatstimulatorymotionissetupevenbyslightdifferences，andthatsense—perceptionisquicktorespondtoit;andfurtherthattheorganwhichperceivescolourisnotonlyaffectedbyitsobject，butalsoreactsuponit.Furtherevidencetothesamepointisaffordedbywhattakesplaceinwines，andinthemanufactureofunguents.Forbothoil，whenprepared，andwinebecomerapidlyinfectedbytheodoursofthethingsnearthem;